Chromizing coating



United States Patent 2,885,301 YCHROMIZING COATING George A. Samuel, White Plains, N.Y., asslgnor to Metal Dlfiusions, Inc., Philadelphia, Pa., a corporation of New Jersey i No Drawing. Application August 2, 1956 Serial No. 601,606

0 1 i2 Claims. c1; 117-42 The present invention relates to chromizing metal parts, particularly iron and steel. a

' A' purpose of the invention is to permit chromizing ina furnace inwhich oxidizing and combustion gases are present, without the need for an inert or reducing atmosphere furnace or an hermetically sealed rctort. p A further purpose is to provide glass forming ingredients in'the chromizing composition which form a protective skin and exclude oxygen partial pressure, thus makingit possible to chromize in a fuel fired furnace, an electric furnace or other suitable furnace without atmospheric control. 7

A further purpose is toprovide a glass forming chromizing skin for coating which is rendered friable and disintegrates on cooling. A'further purpose is to prevent escape of the chromizing catalyst during chromizing. I

further purpose is to permit chromizing in a continuous furnace without an atmospheric control. A further purpose is to permit chromizing powdered iron compactswithout atmospheric control. jFu rther purposes appear in the specification and in the claims. "*In the prior art, chromizing has almost invariably been carried out in a retort or vessel containing an inert or=reducing atmosphere protective from oxygen. This constitutes a serious limitation, as it necessitates use of very expensive equipment, and also in many ,cases re- 2 percent, and which are to be chromized or chromized and sintered.

' In accordance with the invention the parts to be chromized'are coated with the glass forming material which contains a chromizing catalyst, an inert bodying material-and a source of chromium. Unlike the practice carried out in prior box chromizing, the chromizing catalyst V is no longer a high volatile disassociating compound such as ammonium chloride, ammonium fluoride or the like, but is a relatively unvolatile material atchromizing temperature, preferably a chromium halide, most desirably chromium fluoride but less desirable chromium chloride ,'bromide or iodide. The particular halide used may be either the chromous or chromic. The bromide and iodide are less desirable because they are volatile.

The glass forming ingredients will suitably be metal halides such as chloride with desirable addition of sodium chloride to lower the melting point, or they may be powdered glass, with or without materials lowering the melting point such as boric acid or borax alone, sodium oxide or other alkali metal oxide alone, or boric acid or borax with alkali metal oxide. i The inertbodying material may be powdered kaolin; rawor calcine, powdered alumina, raw or calcined, or other suitable refractory;-

The source of chromium will preferably be ferrochrome (preferably the 65 percent chromium grade, low

, carbon) but can equally well be powdered chromium quires packing of the articles in chromizing position in the retort.

,-.Also, because of the use of such equipment, the time ofheating and the total elapsed time of chromizing has been very long.

It; hasnot been possible to chromize in ordinary fuel sible to chromize in any of a wide variety of fuel fired,

I'ectricLbr. other suitable furnaces, without the needifor exclusion of oxygen, and without requiring inert or reducing atmospheres. Furthermore the parts to be chromized can be exposed to the interior of the furnace without packing chromizing compounds around them, heating rates are greatly increased, and the total time of the chromizing cycle is greatly reduced.

According to the invention, therefore, chromizing can be carried out in many variable furnaces which are for the first time made suitable for chromizing. This is true not only with respect to components and parts made from iron, low carbon steel, intermediate carbon steel or even high carbon steel of both plain carbon and alloy grades, and also with respect to powdered iron compacts including those which contain copper of the order of 5 to 10 metal. I

All of the solid ingredients are preferably reduced to a powder finer than 100'mesh.

I Example I Steel parts include A151 1010 steel sheet, hacksaw blades; drill;rods: (AISI 1090), were dipped in a slurry consisting of the following ingredients:

, Grams Barium chloride 300 Sodium chloride Chrornic" fluoride 25 Uncalcined 'kaolin powder 35 Ferrochrome powder This material was mixed with 200 grams of water. The fluoride used was CrF -3.5H,O.

The parts were placed wet in a gas fired furnace at 1950 F. The coating dried immediately and formed a glaze or skin, which was relative impervious to the chromizing vapor, and substantially chromizing was obtained infive minutes, while the chromizing was very pronounced in twenty minutes. The chromium diffusion on high and medium carbon steel parts was 0.0005 inch in five minutes and.-0.001 inch in twenty minutes.

After chromizing theparts were withdrawn from the furnace and allowed to cool, 'and'the' residual coating was wiped off. Tapping of the parts followed by washing in water will also remove the residual powder.

The composition above may vary widely, good results being obtained with a composition as follows:

Percent Barium chloride 45 to 70 Sodium chloride 11 to 20 Chromium fluoride 2 to 10 Inert bodying material 2 to 15 Source of chromium 5 to 25 Experiments indicate that chromizing as above is faster than that obtained in pack chromizing techniques even if allowance be made for the more rapid heating.

Example 11 v Chromizing according to the present invention may be carried out using the following composition:

This mixture is coated on. the parts and they are placed wet or after drying as desired in a furnace at 1900" F. An impervious glass-like skin forms a chromizing case very rapidly as above.

The composition may vary widely, the preferred range being:

Percent Powdered glass f 30 to 60 Boric acid to 15 Sodium carbonate 5 to 15 Chromic fluoride 2 to 25 Raw powdered kaolin 5 to 30 Chrome metal powder 5 to 25 When reference is made herein to boric acid, it will be understood that borax and sodium borate can equally be used. The preferred powdered glass is glass of conventional window or jar glass composition, though special glasses can be used as long as the melting point is brought within the operating range.

The technique of Example I or II may be applied with success to the chromizing and sintering of green powder iron compacts, with or without the presence of for example, 5 or 10 percent of copper.

This offers the advantage of providing sintering of such compacts without the need of atmospheric control and also protecting the surface by chromizing. It also enables the use of continuous furnace without vestibules in chromizing.

It will be evident of course that the slurry can be applied in any suitable way as by dipping, spraying, brushing, roller coating, or otherwise.

In view of my invention and disclosurevariations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention, without copying the process and product shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent:

1. The method of chromizing iron and steel parts, which comprises coating the parts with a composition essentially comprising powdered glass, a source of chro mium, a chromium halide, and a bodying material, and subjecting the parts thus coated to a temperature of between 1600 and 2300 degrees F.

also includes boric acid.

3. The method of claim 1, in which the composition also includes an alkali metal oxide forming material.

4. The method of claim 1, in which the composition also includes boric acid and sodium carbonate.

5. The method of chromizing iron and steel parts, which comprises coating the parts with 45 to 70 percent of barium chloride, from 11 to 20 percent of sodium chloride, from 2 to 10 percent of chromium halide, from 2 to 15 percent of inert bodying material and from 5 to 25 percent of a source of chromium in finely divided form, all the above percentages being by weight on the dry ingredients, and maintaining the parts thus coated at a temperature between 1600 and 2300 degrees F.

6. The method of chromizing iron and steel parts, which comprises coating the iron or steel parts with a composition comprising from 30 to percent of powdered glass, from 5 to 15 percent of boric acid, from 5 to 15 percent of sodium carbonate, from 2 to 25 percent of chromium halide, from 5 to 30 percent of inert bodying material and from 5 to 25 percent of a source of chromium in finely divided form, the above percentages being by weight on the basis of the dry ingredients, and maintaining the parts thus coated at a temperature of between 1600 and 2300 degrees F.

7. A chromizing composition essentially comprising glass powder, a chromium halide, an inert bodying material and a source of chromium in finely divided form.

8. A chromizing composition of claim 7, which also includes boric acid.

9. A chromizing composition of claim 7, which also includes an alkali metal oxide forming material.

10. A chromizing composition of claim 7, which also includes boric acid and an alkali metal oxide forming material.

11. A chromizing composition comprising between 30 and 60 percent of powdered glass, between 15 and 25 percent of boric acid, between 6 and 15 percent of sodium carbonate, between 2 and 25 percent of chromium halide, between 5 and 30 percent of kaolin and between 5 and 25 percent of a source of chromium in finely divided form.

12. A chromizing composition comprising between 45 and percent of barium chloride, between 11 and 20 percent of sodium chloride, between 2 and 10 percent of chromium halide, between 2 and 15 percent of inert bodying material and between 5 and 25 percent of a source of chromium in finely divided form.

References Cited in the file of this patent UNITED STATES PATENTS 928,398 Patterson et al. July 20, 1909 1,729,065 Cole Sept. 24, 1929 1,746,924 Bendix Feb. 11, 1930 1,902,503 Howe Mar. 21, 1933 2,219,004 Daeves Oct. 22, 1940 2,536,774 Samuel Jan. 2, 1951 2,836,513 Samuel May 27, 1958 FOREIGN PATENTS 160,812 Australia Ian. 28, 1955 

1. THE METHOD OF CHROMIZING IRON AND STEEL PARTS, WHICH COMPRISES COATING THE PARTS WITH A COMPOSITION ESSENTIALLY COMPRISING POWDERED GLASS, A SOURCE OF CHROMIUM, A CHROMIUM HALIDE, AND A BODYING MATERIAL, AND SUBJECTING THE PARTS THUS COATED TO A TEMPERATURE OF BETWEEN 1600 AND 2300 DEGREES F. 